Peel the onion and cut into thin slices. Clean, wash and slice the zucchini and eggplants. Wash, clean and cut the tomatoes into eighths. Heat the oil in a pan and fry the onion in it for about 1 minute. Add the zucchini and eggplants and fry for 4-5 minutes over medium heat, turning. Add the tomatoes about 1 minute before the end of the cooking time and let them braise. Wash the rosemary, dab dry and pluck the needles from the twig.
Add just before the end of the cooking time. Season with salt and pepper. Crumble the feta cheese, grate the Gouda finely. Spread both over the vegetables. Gratinate in the preheated oven (electric cooker: 200 °C/ electric cooker: level 3) for 15-20 minutes. Serve garnished with rosemary as desired